Speaking from personal experience, I can vouch for how unnerving it is to have a phone go off at a classical concert. Unfortunately, even setting an iPhone to Airplane Mode is not enough of a safeguard to keep it from making noise. Accidentally press that home button and Siri still comes to life! Not what you want to have happen, trust me.
So in an effort to spare all those hard working musicians and audience members the potential interruption of your phone somehow going off during the concert, please turn your phone off (as in, dead). Thank you.